My and my friend switch with bass and guitar in our band but the problem is his main hobby is skateboarding(he's damn good). Since he skates a lot he doesn't practice very often I know he has the stuff to play well but he would need to practice more often, you see he can't play bar chords and very often has a problem with timing.

I would n't ever kick him out, because is a extremly good lyricist (sp?) and is the only person in the area who shows the same passion for punk and ska music.

How can I motivate him to practice more often?
I try to find people who are motivated in the first place, it's as important to me as skill. Having guys who will commit to practicing is big.

That being said, maybe convert him to vocals if he's such a good lyricist? There's no way he can be forced into doing something he doesn't want to do; I've tried with band members, and it just doesn't work.
Tell him he sucks at skateboarding and just show off a lot, competition will usually want him to be better if he's like that but I don't know too much about him just something my friends would do.
get another guitarist/bassist and keep him as vocalist?
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Just casually point out that the rest of the band is advancing further in terms of skill than him. Don't say it directly to him, say it behind his back but loud enough so he can hear it.

Then he'll be practicing day and night, and won't even mention it.
Forget him, bands are generally self motivated. If they wont practise then go elsewhere. You'll find other lyricists. Are u sure they care about the band that much ? do something to show him you guys are serious, do a gig or something, that usually motivates
Wait, you guys are playing ska music and he cant play barre chords?
You can't. If he doesn't want to practice more than his mind is made up. Trust me if you do try to get him to practice more it'll only annoy him. Found that out the hard way and lost my bassist.
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You can't make him practice.

If you want to nudge him towards practising, make bigger shows of slowing down for him, of playing easier songs for him.

Might not make him practice, but it would at least give him a hint that he's fallen behind.
have him play bass only, its not as hard as guitar and im completely guessing doesn't take as much talent but its still damn hard to be truly good at either
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i second guitar_guy_6006. if he only plays bass then he doesnt have to be awesome anyway.